Polypeptides, including proteins, have become increasingly important in various health related fields. For example, in recent years, peptides have been found to be of pharmacological importance against various diseases, such as cancer, diabetes, plant toxins, and the like. Other polypeptides have shown specific activity as growth promoters or suppressants, antibiotics, insecticides, contraceptives, anti-hypotensives, sleep-inducers, anti-depressants, analgesics, etc. The list is long and varied.
The importance of polypeptides has sparked renewed interest in finding new methods for synthesizing polypeptides from amino acids or from smaller peptides.
A basic problem in peptide synthesis is one of blocking or protecting the carboxy group from interacting with an amino group on the same amino acid and/or preventing functional groups such as an amino group, carboxy group, hydroxy group, mercapto group or amido group on a side chain of an amino acid from undergoing undesirable reactions. The objective is to prevent undesirable side reactions by attaching to an amino acid a group that will render the above-identified substituents unreactive so as to permit the desired reaction to take place. In addition, the blocking group should be easily removed without destruction of any peptide linkage that may have been built during the synthesis.
Amino protecting groups such as FMOC and BOC have been widely used in peptide synthesis, especially solid phase peptide synthesis, including synthesis of peptide amides and peptide acid segments. In the case of protected peptide acids, the segments could be utilized in segment condensation utilizing either solution or solid phase techniques.
One of the methods to effect segment condensation involves the use of .alpha.-FMOC or BOC protection with side chain protection provided by t-butyl based functions (BOC, t-Bu-ether, t-Bu-ester) or benzyl based substituents, respectively. A potentially orthogonal approach involves the use of C-terminal carboxy protection by the allyl ester group with eventual cleavage by palladium catalyzed reactions. Such deblocking reactions are sensitive to traces of oxygen, and may be difficult to reproduce. Far more reliable and simpler to execute, however, is a system whereby a very mild acidic reagent is used to cleave the C-terminal ester linkage.
The present invention provides blocking groups which protect amino groups, amide groups, mercapto groups, hydroxy groups or carboxy groups from undergoing undesirable side reactions, especially during peptide synthesis, provides blocking groups which are easy to remove and provides a system wherein the results are easy to reproduce.
The key to the present invention involves recognition of the enhanced solvolytic behavior of the cyclopropyl methyl substrate. The present invention is directed to the use of the cyclopropylmethyl derivative in protecting various groups on an amino acid or peptide during peptide synthesis. These groups may be located on the main chain or on the side chain. These groups include amide, amino, carboxy, mercapto and hydroxy groups.